This is a Unity/HLSL port of the excellent Annotated Realtime Raytracing blog post by Kevin Fung.

The original GLSL shader runs live in a web browser on ShaderToy. Unity looks the same but at 138fps (Intel HD GPU).

This was much easier than I thought it would be. Here are the changes I made:

• Added HLSL kernel (CSMain) that calls the GLSL fragment shader (mainImage)
• Added iGlobalTime and iResolution shader variables
• Replaced vec2 with float2
• Replaced vec3 with float3
• Replaced vec4 with float4
• Expand constructors with 1 argument to n, e.g. vec3(1.0) -> float3(1.0,1.0,1.0)
• Change const to static const
• Change struct initializer syntax, e.g. Material(vec3(0.0,0.2,1.0),1.0,0.0) -> { float3(0.0,0.2,1.0),1.0,0.0 }

I learnt about Unity Compute Shaders from Coxlin's Blog. This article just multiplies 4 integers by 2.0 in a Computer Shader and prints the results to the console. Everything you need to know to get started.

Understanding the relationship between the [numthreads(x,y,z)] attribute in the shader, and the shader.Dispatch(kernelIndex, gx,gy,gz) call in C#, is fundamental. The kernel CSMain is executed (x*y*z) * (gx*gy*gz) times and the id passed into the kernel ranges from (0,0,0) to (x*gx, y*gy, z*gz).

Here is how the kernel is evaluated for each thread in pseudocode:

 const int numThreadsX, numThreadsY, numThreadsZ; static void kernel(int x, int y, int z) { } static void Dispatch(int threadGroupsX, int threadGroupsY, int threadGroupsZ) { for (var gx = 0; gx < threadGroupsX; gx++) { for (var gy = 0; gy < threadGroupsY; gy++) { for (var gz = 0; gz < threadGroupsZ; gz++) { // Dispatch group for (var x = 0; x < numThreadsX; x++) { for (var y = 0; y < numThreadsY; y++) { for (var z = 0; z < numThreadsZ; z++) { kernel( gx * numThreadsX + x, gy * numThreadsY + y, gz * numThreadsZ + z ); } } } } } } }